Towing a Honda CRV one time - NC to Maine

A few opinions; tow dollies can go faster than 55 and that restriction is likely due to some insurance mandate from Uhaul as a company to rent them. I doubt 90% of the people that rent a tow dolly from U haul maintain under 55. I have personally seen them go much faster. Secondly, while I am always in favor of a supplemental braking system, I think you can get by with this one-time, one way trip. I mean, no one else has a supplemental braking system when renting a uhaul dolly and I've seen some towed with vehicles that would probably really need it. Your MH is big enough I doubt it will matter significantly provided you drive safe and with enough cushion. Does the flatbed trailer rental have brakes? and are you sure about the tongue weight? If the towed car is centered over the axles of the trailer I would guess it's designed to minimize tongue weight. Uhaul should know the specs.
Lastly, if you decided to buy a tow dolly, I doubt you'd be stuck with it for long. When we sold ours people were falling over themselves to buy it.